1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member having a photosensitive layer of the function separated type, and more particularly to an electrophotographic photosensitive member having a charge generating layer which contains a particular resin, as well as an electrophotographic apparatus, a device unit and a facsimile machine using the photosensitive member.
2. Related Background Art
To date, there have been proposed and practiced so-called many electrophotographic photosensitive members of layer-laminated type that are separated in function to a charge generating layer and a charge transporting layer. These photosensitive members are superior in sensitivity, charge retainability and surface strength to the photosensitive members which precede the former and have a photosensitive layer of single layer type (see, for example, U.S. Pat. No. 3,837,851 and U.S. Pat. No. 3,871,882).
In the electrophotographic photosensitive member of the layer-laminated type, charge carriers generated upon light being absorbed by the charge generating layer are injected into the charge transporting layer and moved to the surface of the photosensitive member to neutralize charges on the surface of the photosensitive member, thereby producing an electrostatic contrast. The role borne by the charge generating layer in the above process is very important. Stated otherwise, the charge generating layer affects electrophotographic characteristics to a quite large extent in points such as how many and evenly the charge carriers are generated, how efficiently the generated charge carriers are injected into the charge transporting layer, and how smoothly opposite charge carriers are transported into a support. The charge generating layer usually consists of an organic pigment which serves as a charge generating material and a binder resin as a binding agent. In general, the weight percentage of the binder resin with respect to the organic pigment is not too low, i.e., on the order of 25 to 200 wt. %. It is accordingly thought that the binder resin in the charge generating layer operates as a very serious influence upon movement of the generated charge carriers. Thus, the basic structure, functional group, molecular weight, purity and other properties of the binder resin greatly influence electrophotographic characteristics such as sensitivity, potential level and durability. Judging from prior references and patent publications, however, the binder resin in the charge generating layer has been mainly recognized as merely an aid for the organic pigment as a charge generating material and it is enough if it is able to provide dispersibility, bonding ability and mechanical strength. A butyral resin, for example, is widely used as the binder resin because of good dispersibility for the pigment, but the electrophotographic characteristics of a photosensitive member using the butyral resin are not always satisfactory from the point of residual potential, potential fluctuations and photomemory, taking into account the recent demand for higher image quality. Further, by using a benzal resin produced from substituted or unsubstituted aryl aldehyde and polyvinyl alcohol, as disclosed in Japanese Patent Laid-Open No. 62-30254, there can be obtained better characteristics in sensitivity, residual potential, and photomemory than in the case of using a butyral resin. However, the benzal resin generally has poorer dispersibility than the butyral resin and may cause background fog on an image or picture when used as a binder for the charge generating layer of the photosensitive member for laser beam printers, thus requiring more improvement. Additionally, in the case of using both a butyral resin and a benzal resin in mixed fashion, these resins act independently of each other, which may rather deteriorate the total characteristics.